Hybrid electric vehicles typically include a powertrain having two power sources, which may establish first and second power flow paths to vehicle traction wheels. The first power source is an internal combustion engine with a planetary gear set for distributing power through separate power flow paths to a generator system and to vehicle traction wheels. The second power source is an electric drive system including first and second electric machines serving primarily as a generator and an electric drive motor, respectively. The second power source also includes a battery. The battery acts as an energy storage medium for the generator and the motor.
When the powertrain is operating with the first power source, the engine power is divided between the two flow paths by controlling generator speed. In this fashion, engine speed may be decoupled from wheel speed, such that vehicle speed changes do not depend upon engine speed changes. When the powertrain is operating with the second power source, the electric motor drives the torque output shaft to the vehicle traction wheels through gearing.
Of the two electric machines, the drive motor is most critical for controlling total wheel torque and electric power consumption. As a result, if the drive motor is unavailable, the powertrain capabilities may be severely limited. In such circumstances, known hybrid vehicles will either disable a drive mode or will enter a limited operating strategy (“creep”) mode. Either method is detrimental to customer satisfaction. Consequently, it is desirable to develop a method enabling a hybrid vehicle with an unavailable drive motor to be driven at speeds greater than those allowed in creep mode.